Myosin is a molecular motor binding ATP and actin to produce work by causing relative translation of the two proteins. Myosin contains a lever arm probably executing a power stroke by rotating through an angle of ~70o to translate actin against resistive force. ATP hydrolysis at myosin's active site energizes contraction by influencing lever arm movement and is influenced by allostery with actin in actin-activation of myosin ATPase. The influences are conducted through the protein matrix by coupling pathways investigated by mutation (naturally occurring and computation inspired), molecular dynamics simulation (MD), and structure/function assays. Two coupling pathways identified for study mediate actin-activation of myosin ATPase and conformation change triggering tryptophan nucleotide sensitivity that might link small active site displacements to the larger lever arm movement. The goal of the project is to elucidate the native relationships among actin binding, active site conformation, lever arm rotation and protein displacement and then to observe how these relationships are affected by modifications introduced to coupling pathways. Human skeletal myosin variants play a fundamental role in exercise physiology, human disease, and population diversity. The variants involve widely dispersed amino acid substitutions covering several regions essential to function and are naturally embedded clues to discovering functional domain interconnectedness through the coupling pathways. They implicate sites for mutagenesis in model proteins and are essential for correlation of myosin functional alteration to phenotype. Myosin MD simulation provides complementary insights into how coupling pathways perform. MD introduces the causality test identifying source, path, and termination of coupling networks in sequential time that is an integral part of the competent motor. Causality testing applied to tryptophan nucleotide sensitivity has converged with experimental findings from a tonic smooth muscle myosin to suggest tryptophan nucleotide sensitivity could disconnect from lever arm movement in native myosin. A new experimental causal rotation/displacement metric, quantifying completion of a productive myosin cycle, will correlate myosin lever arm rotation with displacement of a bound actin filament (F-actin) in an in vitro assay. The two-molecule technique utilizes a green fluorescent protein (GFP) on myosin and nanometer resolution localization of a fluorescent probe bound to F-actin. Myosin variants that are, adapted to specialize function, implicated in human disease, or sourced in population diversity, are mined for insight into functional divergence. MD simulation introduces causality to characterize myosin coupling networks and produces experimentally testable hypotheses. A causal two-molecule assay tests completion of a productive myosin cycle and characterizes myosin's ability to displace actin. These analytical tools are next-generation methods addressing transduction and motility in muscle myosin.